Positronium spectroscopy: precise measurement of the ground state hyerfine interval

The hyperfine interval, Δnu, of ground state (n = 1) positronium was measured to high precision yield Δnu = 203.389 10(74) GHz (3.6 ppm). The technique is similar to that used in all previous measurements. Positronium is formed in a gas in a high static magnetic field which causes pronounced Zeeman splitting of the 3S1 state. A microwave field is used to drive transitions between the 3S1 levels. The value of Δnu, the difference between the 3S1 and 1S0 states, is derived by extrapolating the measurement to zero static magnetic field and zero gas density. The present theoretical value for Δnu can be written as Δnu = 203.4003 + a(0.0108) + b(0.0021) GHz, where a and b have yet to be fully calculated. The error in the above theoretical expression is about 50 ppm, the size of the uncalculated terms 0(α4R/sub in equilibrium/c). When all of the terms of this order are calculated, the error should be reduced to a few ppm